The present invention relates to a method for the production of a bulk of molten metal, preferably molten steel in an electric arc furnace, comprising the formation of a foaming top slag with a temperature of 1400-1800xc2x0 C. and the supply of oxygen in the form of oxygen gas and/or other oxygen carriers, e.g. metallic oxides, to the molten metal in order to oxidize at least part of the existing silicon in the melt for heat generation and to oxidize at least part of the carbon in the melt for heat generation and to generate gas in the form of CO and/or CO2 which contributes to the slag foaming, by which the supply of oxygen to the melt also brings about oxidation of metal elements other than silicon in the melt, in this text generally referred to as valuable metal elements, which go into the slag and are reduced there by the addition of reduction agents to the top slag so that these elements to a considerable degree are recovered to the melt.
The invention relates as well to a metallurgical product useable as a doping agent in the production of molten metal, preferably molten steel, in an electric arc furnace to create favourable conditions for the reduction of oxidized, valuable metal elements which have accumulated in the top furnace slag, where the metallurgical product itself participates in the reduction process, contributing to and/or maintain foaming of the top slag as well as giving a surplus of metal to the melt. The invention relates as well to the use of such a metallic product.
Most electric arc furnaces are distinct melting machines, and there is an ongoing search for new means in achieving higher power supply to decrease tap-to-tap times. The thermal loading on the furnace walls and roof during the refining period however limit the maximal effect which can be applied during this stage. By using a foaming slag the radiation from the electric arcs can be shielded, which brings about decreased thermal loading on the surroundings. Further favourable effects are more stable electric arcs and improved heat transfer to the melt. Foamy slag practice according to known techniques comprises the injection of oxygen as well as carbon and/or carbon carriers. Oxygen is injected into the steel to form oxides, which are transferred to the slag, where they are to be reduced by the carbon injected into the slag. CO/CO2 (g) is formed in the slag during the reduction phase, and the gas-slag emulsion forms a foam.
The foaming of slag in general and in electric arc furnaces in particular has been studied theoretically as well as in practical application and has been a well-established technique for many years in modern electric arc furnace steel making. The following references may be mentioned here:
Cooper, C. F. and Kitchener, J. A.; The foaming of molten silicates, J. Iron and Steel Inst., Vol. 193, pg. 48-55, 1959.
Hara, S., Ikuta, M., Kitamura, M. and Ogino, M.,; Foaming of molten slags containing iron oxide, Tetsu-to-Hagane, Vol. 69, pg. 1152-1159, 1983.
Ito, K. and Fruehan, R. J.; Slag foaming in electric furnace steel making, Trans. of the ISS (IandSM), Aug., pg. 55-60, 1989.
Jiang, R. and Fruehan, R. J.; Slag foaming in bath smelting, Met. Trans., B, Vol. 22B, pg. 481-489, 1991.
Zamalloa, M., Warczok, A. and Utigard, T.; Slag foaming during gas injection, Electric Arc Furnace Proc., Vol. 49, Toronto, Canada, pg. 197-204, 1991.
Masucci, P.; Process for using foamed slag in stainless steel production, U.S. Pat. No.: 5,395,420, 1995.
Even though the foaming of slag has become a well-established technique and is used on a large scale in the production of steel in the electric arc furnace, it is, at least considering the production of high-alloy steels such as stainless steels, still associated with a number of problems which have not yet been solved in a satisfactory manner. One problem has to do with the actual formation and maintenance of an active amount of foam, which requires the supply of large amounts of carbon, of which large parts will not participate at all in either the foaming or reduction processes, but rather disappear with slag and off-gases. Another problem relates to the reduction reactions in the foaming slag, which are generally slow, as nucleation sites are scarce. Molten metal does certainly splash up into the slag to a certain degree, but this is insufficient for the generation of nucleation sites to the extent necessary for achieving the desired reduction rate. To avoid unacceptably long tap-to-tap times, therefore, it is usual that the melt is tapped before valuable metals have been reduced back to the desired degree, which implies loss of valuable metals and problems with the handling of remaining slag. To speed up the reduction process, it has therefore been proposed to dope the slag with fine-grained iron carriers such as filter dust or other metallurgical dust ore concentrate, iron carbide, mill scale, dried metallurgical sludge, iron powder and low-phosphorus pig iron chips or NiO, as described in the following references:
Fudala, B; Process for recycling the filter dust in an electric arc furnace for producing steel, U.S. Pat. No.: 5,493,580, 1996.
Frits et al; Process for producing an iron melt, U.S. Pat. No.: 5,611,838, 1997.
Gxc3x6rnerup, M; Studies of Slag Metallurgy in Stainless Steel making, Doctoral Thesis, Div. of Process Metalurgy, Dept. of Metallurgy, KTH,S-100 44 Stockholm, Sweden, 1997.
It has been reported in the above references that the rate of reduction in the slag is increased considerably by the addition of the above-mentioned types of material. Still certain problems remain. One of these has to do with the physical characteristics of the added reduction- and/or doping agents, which can cause a large part of them to disappear before reacting with the oxides in the slag. This implies a cost for lost material, but even more problematic is that a good reproducibility of results in the process, becomes difficult to achieve. The addition of elementary carbon in the form of powder is still a problem as a large part of the material is lost on addition, decreasing the reproducibility of the process. Pig iron in the form of chips, which has also been proposed, and which can be formed as a residual product from splashing, burrs etc. and in the working of pig iron, is usually contaminated and is for this reason unsuitable as a doping agent. Furthermore, it is from a physical point of view unsuitable to use e.g. mill scale and similar product forms where the particles have large area/volume ratios.